1. Field of the Invention
This invention relates to detection and identification of modulation of unknown sources which are at higher frequencies than half of the sampling frequency of the detecting device and which cannot be remotely identified.
2. Brief Description of the Prior Art
It is often useful to examine modulated signals, such as, for example, optical (such as visible or infrared (IR)) energy. Collection of modulated IR data has, in the past, required specialized sensor hardware which can sample at a high rate to ensure that data does not become aliased. Aliasing results from sampling data at an insufficiently fast rate so that it appears at incorrect frequencies. The bandwidth that can be viewed unaliased is half the sample rate or the Nyquist rate. Aliased frequencies are higher frequencies that fold back into the lower frequency area which is half the Nyquist or sampling rate. Aliasing results in ambiguity in determining the actual frequency of the data collected. The frame rates of, for example, standard imaging optical receivers are too low (30 to 60 Hz) to ensure that modulated data from many sources of interest remain unaliased. Scanner limitations make it very difficult to increase forward looking infrared (FLIR) frame rates above 60 Hz. Accordingly, prior art solutions to the problem of aliasing required a unique sensor, independent of the scanning sensor.